ABSTRACT Adherence is essential for the effectiveness of pre-exposure prophylaxis (PrEP) in reducing HIV transmission. Male sex workers (MSW), men who sell sex to other men, can benefit from PrEP because they are at high risk of HIV acquisition and transmission. However, they have difficulties adhering to a daily pill because they commonly have low awareness of their risk and limited knowledge of new prevention tools, and face barriers to access prevention services. Our long-term goal is to advance implementation of PrEP to reduce HIV incidence in key populations in low- and middle-income countries (LMICs). The objective of this R34 application is to prepare for testing innovative user-centered ways to promote PrEP adherence at scale. Our central hypothesis is that adherence to PrEP can be improved among MSW if PrEP is provided for free along with highly-tailored conditional economic incentives (CEI). The specific aims are: Aim 1: To refine the design of PrEP adherence intervention with user-centered conditional economic incentives to maximize sustained adherence behaviors through a user-responsive computerized survey (n=200). We incorporate quantitatively identified preferences for CEIs through a user- responsive computerized survey. We use conjoint analysis to understand preferences for CEI intervention components and how CEIs should be integrated into an optimal combination package to be tested in Aim 2. Aim 2: Measure the extent to which a user-centered CEI intervention can help MSW increase their adherence to free PrEP in a randomized controlled pilot (n=100). Among MSW who accept to take free PrEP, and return at month 1 for a second pill bottle, we will randomize n=100 MSW to either: standard of care (SoC: information, prescription, free PrEP) or CEI (SoC + incentives contingent on sufficiently-high adherence to PrEP). We will assess the primary outcome (biomarker of adherence using scalp hair analysis) at months 3 and 6, as well as secondary outcomes: clinic attendance/retention, medication possession ratio, self-reported PrEP use, and sexual behavioral disinhibition (number of partners, condom use, incident STI). Aim 3: Estimate the preliminary cost-effectiveness of incentives for PrEP adherence to maximize future policy and practice relevance of this promising intervention strategy. Our working hypothesis is that conditional economic incentives for PrEP adherence will be cost-effective in terms of cost per fully- adherent month on PrEP, cost per HIV infection averted, and cost per disability-adjusted life year saved when compared to controls not receiving the conditional incentives. The expected outcome of this R34 is a demonstration that is feasible to implement user-centered CEIs in this context, as well as preliminary efficacy and cost-effectiveness data. The project will have positive impact because it is a critical step toward scaled-up implementation of PrEP in this highly-at-risk population of MSWs in Mexico, with implications for other concentrated epidemics among MSW worldwide.